1. Field of the Invention
The present invention is related to an image processing apparatus, and more specifically, to an image forming apparatus applicable to a copier of electrophotography type to faithfully reproduce a half tone region.
2. Related Background Art
A laser beam printer is a well-known example of conventional apparatus of such type. There exists a laser beam printer which may effect gradation reproduction by using the pulse duration (or width) modulation of image signal with a reference signal, e.g., a triangle wave signal, and then using the obtained pulse duration for the laser radiation time.
Full color images have been increasing in offices as well as in the fields of printing and designing. Following this trend, getting popular is a color copier which may faithfully read and reproduce a color original. The laser beam printer using the pulse duration modulation as described is also applied to the color copier to faithfully reproduce a color tone. The reproduction of half tone becomes more important than in the common black-and-white copier.
FIG. 14 is a block diagram to show a conventional image forming apparatus. The conventional image density control is explained below referring to FIG. 14.
This is an example of laser beam printer, in which an image is formed by laser beam scanning on a photosensitive drum in synchronization with reading of an original.
A CCD 1 reads an original 9 to obtain an analog image signal. This analog signal is amplified up to a predetermined level by an amplifier 2. The amplified signal is converted by an A/D converter 3 into a digital image signal of eight bits, i.e., 0 to 255 gradations. The digital image signal then passes through a gamma converter 10, which consists of a 256 byte RAM and effects digital conversion in a form of the lookup table, to perform the gamma control. After the gamma control in the converter 10, the signal is input into a D/A converter 14.
The digital signal is converted again into an analog signal. This analog signal is compared at a comparator 16 with a signal of a determined period generated by a triangle wave generation circuit 15 to receive the pulse-width modulation. The binary image signal after the pulse-width modulation is input directly into a laser drive circuit 17, and is then used as a signal to control the radiation of a laser diode 18. The laser radiation from the laser diode 18 is scanned by a well-known polygon mirror 19 in a primary scan direction, passes an f/.theta. lens 20 and a reflection mirror 21, and is impinged upon a photosensitive drum 22 of image carrier revolving in the direction of arrow as shown to form an electrostatic latent image thereon.
The drum 22 is uniformly discharged by an exposing device 28 and then uniformly charged with negative charge by a charger 23. After the charging, the drum 22 receives the laser radiation to form thereon the electrostatic latent image in correspondence with the image signal. The development is conducted by the so-called image scan method often used in the laser beam printer, in which a portion to be developed, or black pixel, is exposed. Thus a developer 24, using the well-known reversal development, supplies toner having negative charge property onto the portion of the photosensitive drum 22 which has been irradiated by the laser beam, to form a real image.
FIG. 15 shows a relation between the surface potential on the drum 22 and the development contrast upon the reversal development. V.sub.D represents a negative potential uniformly charged by the charger 23 as shown in FIG. 14, V.sub.00 a surface potential on the photosensitive drum when a digital image signal of 00.sub.Hex, level 0, given by the A/D converter 3 drives the laser to irradiate the drum 22, and V.sub.FF the surface potential by a digital signal of FF.sub.Hex, level 256. If a development bias V.sub.DEV is applied onto the developer 24 as shown in FIG. 14, the development is effected under the development contrast of .vertline.V.sub.DEV -V.sub.XX .vertline. as shown in FIG. 15.
Supposing the contrast .vertline.V.sub.DEV -V.sub.FF .vertline. is a proper contrast potential (V.sub.cont) and the development density is D.sub.max for the development under V.sub.cont, suitable adjustment of V.sub.cont leads to a preferable image density, generally between 1.2 and 1.8 for the electrophotography.
In FIG. 15, the potential (V.sub.back) is for completely removing fog on a white background of image irradiated by 00.sub.H light quantity.
The developed image or negatively charged toner image formed on the photosensitive drum by the above method is transferred onto a transfer medium, usually a paper, 26 by a transfer charger 25. The residual toner on the drum 22 is scratched off by a cleaner 27. The sequential processes are then repeated.
The conventional image density control in the above-described laser beam printer employs the following two controls in order to obtain a linear image density for the digital image signal of 0 to 255.
One is a potential control to gain the proper contrast potential V.sub.cont defining the maximum density D.sub.max corresponding to the digital image signal of FF.sub.Hex. The other is a gradation control using the gamma converter to control the half tone density corresponding to the digital image signals of 00.sub.Hex to FF.sub.Hex.
The potential and the gradation controls are described below.
The potential control is first explained with reference to FIG. 16.
In FIG. 16, the abscissa represents a grid bias potential of a primary charger and the ordinate a surface charge potential of the OPC photosensitive drum. The unrepresented primary wire is under the constant current control. The line V.sub.D in FIG. 16 shows a charge potential of the photosensitive drum against the grid bias. The line V.sub.00 shows reduction of potential when the light quantity of 00.sub.Hex is irradiated at each point on the line V.sub.D. Similarly, the line V.sub.FF shows reduction of potential when the light quantity of FF.sub.Hex is irradiated. The linearity of lines will be held after long use of the photosensitive drum.
Using the above property, the grid bias may be uniquely determined so that the value (V.sub.00 -V.sub.FF) is equated with the value (V.sub.cont +V.sub.back) as shown in FIG. 15.
Such potential control may correct the difference in sensitivity property of the photosensitive drum among different manufacturing lets and the deterioration of charge capacity with time to always provide a constant V.sub.cont.
The conventional gradation control is explained next referring to FIGS. 17, 18, and 19.
FIG. 17 shows an output image density against the digital image data of 0 to 255 without gradation correction. This property is not linear as known, but an S-shaped curve.
The function of the curve may be linearized by multiplying an inverse function thereof. That is, the inverse function as shown in FIG. 18 can be applied. FIG. 18 is normalized by 0 to 255. The inverse function of FIG. 18 is stored as conversion coefficients in the gamma converter as shown in FIG. 19. The digital image signal is converted in a manner as seen in FIG. 19.
The above gradation control may correct and linearize the function of nonlinear electrophotographic property of FIG. 17.
However, since the above conventional example uses V.sub.cont defining the output maximum density for the potential control, the density of half tone can vary under the identical maximum density as seen in FIG. 20. This may occur even if the above-described gradation control is used, as explained below.
There is a method proposed to control the maximum density and the gradation linearity by making changeable, if necessary, V.sub.cont and the conversion coefficients in response to variable factors in electrophotography. The maximum density shows one-to-one correspondence to the development contrast secured on the photosensitive drum by the light quantity FF.sub.Hex, so that it may be controlled. It is, however, very difficult to control the gradation or half-tone density because of difficulty to detect and control all variable factors as explained below. Thus such problem that the half-tone density may change under the identical maximum density remains unsolved.
The major variable factors in electrophotography are as follows.
The first factor is an environment dependency, for example, a change in sensitivity property of the photosensitive drum depending on the humidity.
The second is a durability dependency, for example, a change in sensitivity property of the photosensitive drum due to abrasion of the drum.
The third is a distribution of machine performance, for example, a change in laser light quantity property for 00.sub.Hex to FF.sub.Hex within a range of distribution of laser units.
As explained before, V.sub.cont and the conversion coefficients of the gamma conversion could be made variable to correct the variable factors. It is, however, more difficult actually to control the gradation to be linear than to control D.sub.max to be constant.
It should also be noted that the visual sensitivity is generally high at the half tone area rather than at the high density portion. Consequently the gradation change rather than the D.sub.max change shows great influence on change in quality of image. If the linearity of gradation is broken by some of the above reasons, such a problem remains unsolved that the preferable gradation or color reproduction cannot be attained, for example, by a copier necessitating especially high gradation reproducibility and by a full color copier requiring high color reproducibility.
U.S. Pat. No. 4,352,553 (Hirahara) discloses a technique to control the image forming conditions directly from the frequency distribution of density of original image. This technique fails to attain the optimum control, missing the correlation between the original density and the image forming density.